With their miniature size, non-invasive and rapid detection, a waveguide Bragg grating refractometric sensor can be a useful tool for chemical analysis and biomedical testing, where the evanescent field of the fundamental mode propagating in a waveguide containing a Bragg grating can interact with liquids and gases surrounding the waveguide in order to measure small changes in the refractive index of the surrounding liquid. Measurements from Bragg grating based refractometers however often can be distorted by fluctuations in the ambient temperature. For temperature insensitive sensors based on fiber Bragg gratings, several techniques have been proposed to discriminate between Bragg resonance spectral shifts associated with refractive index measurements and those induced by fluctuations in temperature. These techniques are implemented by using: a second Bragg grating in a side-polished fiber Bragg grating refractometer as taught by Schroeder et al. in Meas. Sci. Technol. 12, p. 757-764, 2001 and Pereira et al. in Opt. Eng. 43, p. 299-304, 2004 incorporated herein by reference; higher order modes in an etched-core of a fiber Bragg grating sensor as taught by Chryssis et al. in IEEE Quan. Electron. 11, pp 864-872, 2005 incorporated herein by reference; a single sampled fiber Bragg grating possessing the responses of the core guided Bragg grating and cladding guided long period grating as taught by Shu et al. in Opt. Lett. 26, 774-776, 2001 incorporated herein by reference, and; cladding modes in a tilted fiber Bragg grating sensor as taught by Kang, et al. in IEEE Photon. Tech. Lett. 10, pp. 1461-3, 1998; Chan et al. Appl. Opt 46 pp. 1142-1149, 2007, and Laffont et al. in U.S. Pat. No. 7,184,135, also incorporated herein by reference. To increase the interaction of the surrounding medium with the evanescent field about the waveguide core, the fiber cladding is often removed at the location of the Bragg grating. Etched fibers either as a side polish or symmetric tapered etch about the core are taught by Asseh et al. in Fib. Int. Opt. 17 pp. 51-62, 1998, incorporated herein by reference.
These prior art gratings are instructive and provide a useful function, however they are known to suffer from some limitations in terms of the spatial accuracy of the temperature measurement and the mechanical strength of the device. For the case of dual gratings as taught by Schroeder et al., the temperature monitoring grating and the index monitoring grating are at different locations within the fluid being measured. However, it is more accurate to have both measurements performed at the same location. The requirement to improve the sensitivity of the fiber grating refractometers by polishing and etching as taught by Asseh et al. leads to devices that are mechanically weak and fragile. The use of tilted fiber gratings improves the mechanical strength of the grating refractometer and simultaneous monitoring of the induced core and cladding mode resonances can be used to monitor both temperature and index changes at the identical location within the fluid. Because of the flexible nature of optical fibers however, the tilted grating refractometers suffer from the limitation that the cladding mode resonances, which are used to monitor the temperature, are sensitive to physical changes across the fiber cross-section such as shear strains arising from fiber bending.
Another technique for simultaneous measurement of temperature and refractive index with a fiber grating is by the use of a single sampled fiber Bragg grating as taught by Shu et al. in Opt. Lett. 26, 774-776, 2001, incorporated herein by reference. The sampled fiber grating possesses properties that result in both core guided Bragg grating reflectivity responses and cladding guided long period grating responses. The long period grating response is much more sensitive to external refractive index changes than the core Bragg grating response while the core grating response is much more sensitive to temperature. Aside from the limitations resulting from fiber flexibility as stated for the tilted grating refractometer, the sampled fiber Bragg grating refractometer has the additional limitation that the spectral response of the cladding guided long period grating resonance is spectrally broad (>10 nm) making high accuracy measurements of small wavelength shifts difficult. The difference in temperature sensitivities between the core guided Bragg grating and the cladding guided long period grating also need to be taken into account.
The inherently robust nature of integrated optical devices such as planar waveguides make them better suited for refractometer devices than optical fiber from a mechanical stability perspective. In addition other functionalities can be incorporated into planar waveguide. Incorporation of corrugated Bragg gratings that are etched into the core of buried waveguides once the cladding is removed by chemical etching can be achieved as shown by Veldhuis et al. in Pure Appl. Opt. 7, L23-L26 (1998) incorporated herein by reference. Direct UV laser inscribed waveguides and superimposed UV induced gratings were used to fabricate a refractometric device by etching away the top cladding layer on a buried UV-induced waveguide at the location where a grating was UV laser induced as demonstrated by Sparrow et al. in 17th International Conference on Optical Fibre Sensors, Proc. of SPIE Vol. 5855, p. 888-891 (2005) and Emmerson et al. in Appl. Opt. 44, 5042-5045, (2005) incorporated herein by reference.
These prior art planar waveguide gratings provide a useful function, however they are known to suffer from some limitations. In order to measure the refractive index of a fluid, removal of a portion of the surface cladding layer by chemical etching is required in order to access the waveguide core. In the case of Veldhuis et al. an etch process is also required for the Bragg grating fabrication which is an exacting labour intensive process. In Meas. Sci. Technol. 17, pp. 1752-1756, 2006, Dai et al. teach a technique for manufacturing a highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes in the refractive index of a surrounding liquid was developed. By using a ridge waveguide with a small core that is absent a top cladding layer, the evanescent field interaction of the guided mode with the liquid analyte was enhanced. The ridge waveguide is more easily fabricated than the planar waveguide structure as the processing step of adding a top cladding layer is removed. The average sensitivity measured via a shift in the resonance wavelength of the Bragg grating was as high as 1 pm of wavelength shift for a change of 4×10−5 in the refractive index on the liquid layer over top the ridge waveguide core. However, the device was sensitive to temperature change as the Bragg wavelength shifted with temperature (˜11 pm/° C. in silica waveguides). It was also found that the refractometer could be made more sensitive by having narrower ridge waveguide structures.
It is an object of this invention to overcome the aforementioned limitations of the prior art fiber Bragg grating based refractometers, mainly reduction in mechanical reliability to achieve improved sensitivity and thermal stability. In this invention a technique for creating a substantially temperature insensitive refractometer that utilizes core and cladding modes and/or polarization dependent TE and TM waveguide modes in an open-top ridge waveguide architecture absent a cladding layer in order to discriminate between changes in temperature and refractive index is disclosed.